|Publication number||US7547196 B2|
|Application number||US 11/228,109|
|Publication date||Jun 16, 2009|
|Filing date||Sep 16, 2005|
|Priority date||Sep 17, 2004|
|Also published as||CA2580626A1, CA2580626C, US20060067834, WO2006034197A2, WO2006034197A3|
|Publication number||11228109, 228109, US 7547196 B2, US 7547196B2, US-B2-7547196, US7547196 B2, US7547196B2|
|Inventors||LeMoyne Boyer, Doneil M. Dorado|
|Original Assignee||Lufkin Industries, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (3), Referenced by (1), Classifications (12), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This Non-Provisional Application is based on Provisional Application 60/611,148 filed on Sep. 17, 2004 and claims the benefit of that filing date.
1. Field of the Invention
This invention relates in general to control of rod pumped wells and in particular to control of rod pumping equipment for conditions where heavy crude oil production creates viscous and rod drag forces that cause the rod string to fall slower than the pumping unit motion on the downstroke.
2. Description of the Prior Art
When heavy crude oil production creates viscous and rod drag forces that cause the rod string to fall slower than the pumping unit downstroke motion, the pumping unit equipment can be damaged resulting in excessive maintenance costs and reduced production. A prior solution to that problem has been to install a variable frequency drive on the pumping unit and to manually slow the motor speed so that the pump speed is slowed to minimize rod float induced events. The problem with this prior approach is that well conditions change. For example, where heavy crude oil is being produced, cyclic steam injection, steam assisted gravity drainage (SAGD) and other secondary recovery operations require that steam be injected in the well for a time period, followed by pumping the well for a period of time to recover water and heavy crude oil. Well head temperatures change with time, and ambient temperature conditions affect flowline pressures which can adversely affect the rod-pump system with respect to rod float, rod loading and other operational conditions.
3. Identification of Objects of the Invention
A primary object of the invention is to provide Rod Float Mitigation (RFM) methods to detect rod float during rod pumping operations and to control the rod pumping apparatus to mitigate damage to the equipment while maximizing production.
The object identified above as well as other advantages and features of the invention are incorporated in a well pumping controller for a rod pumping system which includes a variable frequency drive (VFD). According to a first embodiment of the invention (called fixed speed option), a rod float condition is sensed by measuring rod load. A controller is provided to compare rod load with a programmed fixed value, and if the rod load falls below the programmed fixed value, then the speed of the VFD is reduced to a preset or fixed value.
According to a second embodiment (called fixed torque option) of the invention, a rod float condition is sensed as in the first embodiment, and when rod float is sensed by the controller, VFD speed is adjusted with a control signal such that the calculated net gear box torque does not exceed a programmed fixed torque limit.
According to a third embodiment of the invention (called variable torque curve option), a controller is activated only when the rod load falls beneath a predefined minimum load. When that condition is sensed, the controller commands the VFD to follow a RFM torque curve on the downstroke. The RFM torque curve is based on the pumping unit geometry and existing crank counterbalance of the pumping unit. This method of controlling the speed of the pumping unit minimizes the amount of speed droop needed to mitigate the rod float condition thereby optimizing production.
Detection of rod float can be obtained by means other than a direct rod load measurement. A proximity switch to detect separation of the carrier bar from the polished rod clamp may be used although such an arrangement may be less successful in practice due to the strict alignment required of a proximity switch. Another way to measure rod float is a direct position measurement of the polished rod and pumping unit carrier bar or related member. Such measurement may be accomplished by means of string position transducers, etched encoder position codes on the polished rod with corresponding sensor, etc.
A rod string 36 of sucker rods hang from polished rod 32 within a tubing string 38 located in a casing 40. Tubing 38 can be held stationary to casing 40 by an anchor 37. The rod string 36 is connected to a plunger 42 of a subsurface pump 44. Pump 44 includes a traveling valve 46, a standing valve 48, and a pump barrel 50. In a reciprocating cycle of the structure, including the walking beam 24, wire rope bridle 30, carrier bar 31, polished rod 32, rod string 36, and a pump plunger 42, fluids are lifted on the upstroke. When pump fillage occurs on the upstroke between the traveling valve 46 and the standing valve 48, the fluid is trapped above the standing valve 48. Most of this fluid is displaced above the traveling valve 46 when the traveling valve moves down. Then, this fluid is lifted toward the surface on the upstroke.
Rod float, also known as rod hang-up or carrier-bar separation, occurs when the polished rod 32 falls slower than the downward motion of the horsehead 28, wire rope bridle 30, and carrier bar 31. Rod float occurs largely due to excessive viscous and rod drag friction forces along the rod string 36 and in the pump 44. It is a result of pumping heavy crude at temperatures where the viscosity is high.
Since the bridle 30 is of the wire rope type, slack occurs usually resulting in separation between the carrier bar 31 and the clamp 29 at the top end of the polished rod 32. When slack exists in the bridle 30, the axial load in the polished rod 32 is zero.
The carrier bar 31 includes a clamping arrangement to retain the polished rod 32, but usually allows for relative linear movement. Thus the rod float event does not normally cause a catastrophic failure in the system, but significant mechanical stresses can occur when the polished rod 32 is once again picked up by the carrier bar 31, ending the rod float event. Likewise, the horsehead 28 generally includes a device to retain the bridle 30 to keep it on the face track of the horsehead 28 in the event slack occurs.
A description of three methods for mitigating rod float for a rod pumping system follows.
When software in the controller 52 (see
When software in the controller 52 senses a low load signal from the surface card (e.g., loads below 200 lbs.), a digital output is sent via signal path 9 to the VFD 8, which may activate a rod float mitigation procedure in software in the VFD 8 according to a second embodiment. Net gear box torque is a function of the motor speed and geometry of the mechanical linkage between motor 12 and the rod pump assembly, 32, 36, 42. VFD speed control to the motor is adjusted such that the calculated net gear box torque will not exceed a programmed fixed torque limit as is illustrated in
Alternatively, software in the controller 52 can detect the low load condition and adjust the command speed being sent to the VFD 8 via lead 9 so that the torque limiting condition is maintained. This can be accomplished by calculating torque within the controller 52 since it has signals representative of the polished rod load (from load cell 33) and stored information about the geometry and counterbalance of the pumping unit. Alternatively, the controller 52 obtains the VFD 8 calculated torque as an analog output via signal path 9 and adjusts the speed being sent to the VFD so that the torque limit is maintained.
According to a third embodiment of the invention, a method is incorporated in software of the controller of
T counterbalance =M*sin(Θbottom of stroke +RK*(Θoffset+τ))
T net gb (at slow speed shaft) =Tmotor*NREV ref
Torque curve rod float control is accomplished by the controller 52 sending a digital output pulse via signal path 9 at the bottom of stroke (and optionally a second digital pulse is sent also at the top of stroke, for improved position detection) which the VFD 8 monitors. The VFD 8 uses its internal motor model to estimate motor 12 rpm and subsequently pumping unit angle (position). The VFD 8 alternatively utilizes its own rpm input to directly measure pumping unit angle.
When the controller 52 senses a low load input (e.g., loads below 200 lbs.) from the surface card (See
If Tnet gb(at slow speed shaft) on the downstroke approaches within a threshold amount of the Tcounterbalance (this could be a percentage or actual value, e.g. if Tnet gb>=95%*Tcounterbalance or if ((Tcounterbalance−Tnet gb)<=20,000 in-lbs), then the drive 8 is programmed to control the speed of motor 12 to try to maintain the net gearbox torque at the threshold value, while the low load signal digital output is active. The Rod Float Mitigation (RFM) algorithm is only active when the pumping unit is on the downstroke and the rod load is below the programmed load threshold. This calculated torque curve limit is illustrated in
As in the second embodiment, an alternative approach is to have the controller 52 detect the low load condition and adjust the command speed being sent to the VFD 8 via signal path 9 so that the torque limiting condition is maintained. This is accomplished by calculation of torque within the controller 52, because it has stored information regarding the polished rod load, geometry and counterbalance of the pumping unit.
Another alternative means of control for the controller 52 provides that it obtains the VFD 8 calculated torque as an analog output via signal path 9 and adjusts the speed being sent to the VFD 8 so that the torque limit is maintained.
Effects of system inertia have been neglected in the embodiments described above. Indeed during normal operation, the pumping unit speed is relatively constant and inertia effects are minimal. However, during the transient speed changes prescribed in the above embodiments inertia effects should be taken into account in the embodiments described above. Because system inertia influences dynamic torques when the unit is decelerating or accelerating, it may be necessary to further reduce the torque limit while the pumping unit is being decelerated. Likewise it may be necessary to increase the torque limit upon acceleration. The rotary inertia torque is added/ subtracted to the programmed fixed torque limit in the second embodiment, or to the programmed threshold limit as described in the third embodiment. The value of this rotary inertia torque is equal to the product of the system inertia (usually referred to the slow speed gear box shaft) and the angular acceleration. A similar procedure can be followed if it is desired to account for the articulating inertia effect. However it is usually much smaller than the rotary effect.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US4971522 *||May 11, 1989||Nov 20, 1990||Butlin Duncan M||Control system and method for AC motor driven cyclic load|
|US6414455 *||Apr 3, 2000||Jul 2, 2002||Alvin J. Watson||System and method for variable drive pump control|
|US6506030 *||Mar 19, 2001||Jan 14, 2003||Air Products And Chemicals, Inc.||Reciprocating pumps with linear motor driver|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US9617837||Jan 14, 2014||Apr 11, 2017||Lufkin Industries, Llc||Hydraulic oil well pumping apparatus|
|U.S. Classification||417/22, 417/42, 417/53|
|Cooperative Classification||F04B2203/0209, F04B49/065, F04B2203/0204, F04B2203/0207, F04B2201/121, F04B47/02|
|European Classification||F04B49/06C, F04B47/02|
|Sep 16, 2005||AS||Assignment|
Owner name: LUFKIN INDUSTRIES, INC., TEXAS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:BOYER, LE MOYNE;DORADO, DONEIL M.;REEL/FRAME:017584/0712
Effective date: 20050916
|Nov 8, 2012||FPAY||Fee payment|
Year of fee payment: 4
|Aug 7, 2014||AS||Assignment|
Owner name: LUFKIN INDUSTRIES, LLC, TEXAS
Free format text: CHANGE OF NAME;ASSIGNOR:LUFKIN INDUSTRIES, INC.;REEL/FRAME:033494/0400
Effective date: 20130826
|Dec 16, 2016||FPAY||Fee payment|
Year of fee payment: 8